Storage system for multi-site remote copy

ABSTRACT

Information about remote copy target volumes of other storage systems  2  each having one and the same original volume V 1  is registered for each remote copy target volume in advance. When there occurs a failure in a copy source storage system  2 , a copy source volume for remote copy target volumes using the storage system  2  having a failure as their copy source is selected newly from the registered copy source volumes, and remote copy is resumed.

The present application is a continuation of application Ser. No.10/378,644, filed Mar. 5, 2003, the contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a technique for duplicating databetween storage systems without intervention of any host.

As a technique for avoiding data loss in a storage system due todisaster or the like, there is a remote copy for duplicating data in astorage system at a remote place. Here, assume that the storage systemincludes a storage control unit and a storage unit. In addition, theremote copy is a technique in which one storage system transfers data inits own storage system to another storage system at a remote place, andstoring the data in a storage unit in the remote storage system. As forthe remote copy, U.S. Pat. No. 6,408,370 discloses a technique forcopying data between different storage control units withoutintervention of any host computer (hereinafter referred to as “host”).

In recent years, there is a demand for performing remote copy amongthree or more sites. Here the word “site” means a place where a host, astorage system and the like are installed.

As for remote copy among three sites, U.S. Pat. No. 6,209,002 disclosesa technique in which data received from a first site is transferred in alump to a third site by a second site. According to this technique, evenif data in the second site is updated by the first site a plurality oftimes, finally updated data may be sent to the third site in datatransmission. It is therefore possible to suppress the increase of thecommunication cost in transfer paths among the sites.

SUMMARY OF THE INVENTION

In order to manage remote copy among N (N≧3) sites for avoiding dataloss, it is desired to adopt a form in which when one of the N sitessuffers from a natural disaster or the like, remote copy isre-constructed out of the surviving N−1 sites immediately, and theremote copy is kept up.

However, in the conventional art, change of a copy source in case thatone site suffers from disaster is not taken into consideration. For thisreason, there is a problem that when remote copy among storage systemsusing the storage system of the suffered site directly or indirectly asa copy source is shut down, the remote copy cannot be resumed till thesuffered site is restored.

To solve this problem, a storage system accepting data transfer performsprocessing for switching a copy source of data to another storage systemwhen there occurs a failure in a storage system as the copy source fortransferring the data to the storage system accepting data transfer. Onthat occasion, the storage system selects a storage system registered inadvance in a management information memory, as a new copy source.Whether transfer of data is kept up or not may be determined inaccordance with the priority of the data or the priority of the new copysource.

Incidentally, the copy source is here defined as a storage systemtransferring data to another storage system in remote copy. When data ina storage system A is transferred to a storage system B, an expressionsuch as “a storage system B using a storage system A as a copy source”or “a volume C belonging to a storage system B using a storage system Aas a copy source” is used. In addition, a copy destination is defined asa storage system having a volume in which data transferred from anotherstorage system should be stored in remote copy. Such an expression isused that “a storage system B is set as a copy destination of a storagesystem A” or “a storage system B is set as a copy destination of avolume C belonging to a storage system A”. Further, the word “directly”means that the storage system B receives data from the storage system Awithout intervention of any other storage system, and the word“indirectly” means that the storage system B receives data from thestorage system A through one or more storage systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of the configuration of remotecopy among N sites;

FIG. 2 is a diagram showing an example of the configuration of a storagesystem;

FIGS. 3A and 3B are diagrams showing an example of the relationshipbetween sequence management information and a cache memory;

FIGS. 4A and 4B are diagrams showing an example of volume managementinformation;

FIG. 5 is a diagram showing an example of write processing executed by astorage system;

FIG. 6 is a diagram showing an example of asynchronous transferprocessing executed by a storage system which is an asynchronous copysource;

FIG. 7 is a diagram showing an example of asynchronous formalizationprocessing executed by a storage system which is an asynchronous remotecopy destination;

FIG. 8 is a diagram showing an example of the outline of processing inrespective storage systems for remote copy among N sites;

FIG. 9 is a diagram showing an example of failure processing executed bya storage system having detected a failure in a copy source;

FIG. 10 is a diagram showing an example of remote copy suspensionprocessing executed by a storage system having detected a failure in acopy source;

FIG. 11 is a diagram showing an example of remote copy resumptionprocessing executed by a storage system having detected a failure in acopy source;

FIG. 12 is a diagram showing an example of a differential copy programexecuted by a storage system set as a new copy source by a storagesystem having detected a failure in a copy source; and

FIG. 13 is a flow chart showing an example of procedure for switching acopy source to a new copy source and resuming remote copy by use of thenew copy source.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a configuration diagram to which the present invention hasbeen applied. In each site, a host 1, a storage system 2, etc. areconnected to one another through a local network T2 such as a LAN (LocalArea Network) or a SAN (Storage Area Network). On the other hand, of thestorage systems 2 in the respective sites, the storage systems 2executing remote copy are connected with one another through a globalnetwork T1. The global network T1 is typically a public communicationline, which is often used at a charge by lease from an agency providingcommunication service. However, the configurations of the local networkT2 and the global network T1 are not provided for limiting the presentinvention. When attention is paid to data of one volume and copydestinations thereof, FIG. 1 shows the state where data is transferredfrom a site 1 to a site (K−1) and a site N through a site 2, and to asite (N−1) through a site K.

The storage system 2_1 in the site 1 has an original volume V1 storingdata written directly from the host 1. Each of the storage systems 2_2and 2_k in the site 2 and the site K has an intermediate volume V2 forstoring data transferred from another storage system 2, and storing datato be further transferred to another storage system. Each of the storagesystems 2_(k−1), 2_N and 2_(N−1) in the site (K−1), the site N and thesite (N−1) has a terminal volume V3 for storing data transferred fromanother storage system 2 and not to be transferred to any other storagesystem 2. Here, the word “volume” means a unit by which a storage mediumsuch as a magnetic disk or the like is handled, and the volume may be alogical unit or a physical unit.

The operation of remote copy is roughly divided into two. One of them isa synchronous remote copy for transferring data to an secondary storagecontrol unit before a termination report of write processing carried outin response to a write request from the host 1 or another storage system2. The other is an asynchronous remote copy for transferring data to ansecondary storage control unit asynchronously with a write request aftera termination report of write processing. In the asynchronous remotecopy, a volume in an secondary storage control unit has to be updated inaccordance with the order in which a volume in a primary storage controlunit is updated, in order to secure the update order of a log for anapplication of a database or the like. To attain this, in theasynchronous remote copy, there is a method in which data received fromthe principal storage control unit is stored in a volume (or cachememory 5) in the secondary storage control unit in accordance with itssequence number as will be described later. This method is calledformalization. Here, the principal storage control unit designates astorage control unit of a copy source to transfer data therefrom, andthe secondary storage control unit designates a storage control unit ofa copy destination to transfer data thereto from the principal storagecontrol unit.

FIG. 1 includes the relationship “one to many” of the site 2 and thesite K with respect to the site 1, and the relationship “multi hop” ofthe site 1 to the site (K−1) through the site 2. The delay till datareaches the copy destination having a terminal volume from the copysource having an original volume is smaller in “one to many” than in“multi hop”. Therefore, the data in each copy destination can be keptcloser to the data stored in the copy source in “one to many”. However,the number of copy destinations to which the copy source transfers datais so large that the load on the copy source increases.

FIG. 2 shows the configuration of a storage system 2. The storage system2 includes a storage control unit 21, a storage unit 22, and amaintenance terminal 23 for giving a notification to the storage controlunit 21 or displaying the internal state of the storage control unit 21.However, the maintenance terminal 23 is not essential. In FIG. 2, thesoftware is encircled.

The storage control unit 21 includes a channel interface 3, a diskinterface 4, a cache memory 5, and a management information memory 6.Incidentally, through not shown, there are a plurality of channelinterfaces 3 and a plurality of disk interfaces 4. In addition, thecache memory 5 and the management information memory 6 are duplicatedrespectively.

Incidentally, according to another embodiment, one CPU for performingoverall control over the channel interface 3, the disk interface 4, thecache memory 5, the management information memory 6 and so on may bedisposed in the storage control unit 21 in place of CPUs 30 and 40disposed in the channel interface 3 and the disk interface 4respectively.

The channel interface 3 and the disk interface 4 are connected to thecache memory 5 and the management information memory 6 throughinput/output (I/O) portions 38 and 41 by path respectively. Theconnection by path may be implemented by switch-connection orbus-connection, but the present invention is not limited thereto.

The channel interface 3 is connected to the host 1, a copy source or acopy destination through an I/O portion 37, the local network T2 or theglobal network T1.

A write program 31, a remote copy suspension program 32, an asynchronoustransfer program 33, a volume notification transfer program 34, adifferential copy program 35 and a remote copy resumption program 36 arestored in a memory 300 in the channel interface 3.

The CPU 30 in the channel interface 3 executes the write program 31 soas to perform write processing in response to a write request from thehost 1 or a write request from a copy source.

The CPU 30 executes the remote copy suspension program 32 and the remotecopy resumption program 36 so as to perform processing for switching acopy source to another storage system 2 registered in advance when thecopy source has suffered from disaster. The method for registeringanother storage system 2 as a copy source will be described later.

The CPU 30 executes the asynchronous transfer program 33 so as toperform processing for transferring data to a copy destinationasynchronously with a write request when the storage system 2 is a copysource of asynchronous remote copy.

The CPU 30 executes the volume notification transfer program 34 so as tonotify a copy source of the contents of processing on a remote copytarget volume of the storage system 2. Here, the remote copy targetvolume means a volume whose data should be copied and saved in anotherstorage system 2. In addition, the processing on the remote copy targetvolume means processing such as remote copy suspension, remote copyresumption, differential copy termination, and the like, as will bedescribed later. The differential copy means remote copy of differentialdata as will be described later.

When any site has suffered from disaster and remote copy is resumedamong the surviving sites, the CPU 30 executes the differential copyprogram 35 so as to perform processing for matching the volume contentsamong the surviving sites.

The disk interface 4 is connected to the storage unit 22 through an I/Oportion 42. In addition, an asynchronous formalization program 41 isstored in a memory 400 in the disk interface 4. When the storage system2 is a copy destination of asynchronous remote copy, the CPU 40 in thedisk interface 4 executes the asynchronous formalization program 41 soas to perform processing for formalizing data and storing the formalizeddata into the storage unit 22.

The cache memory 5 is a nonvolatile memory for storing data. The cachememory 5 stores data to be read and written by the host 1. Theconfiguration of the cache memory 5 can be roughly divided into a normalarea and a temporary area. These areas will be described later.

The management information memory 6 is a nonvolatile memory for storingmanagement information of data. The management information memory 6stores sequence management information and volume management informationrequired for remote copy between storage systems 2. Incidentally, theinformation stored in the management information memory 6 may be storedin the cache memory 5.

FIGS. 3A and 3B show an example of the details of the sequencemanagement information. Each row in FIG. 3A will be hereinafter referredto as “sequence management information entry”. The sequence managementinformation is information to be used in asynchronous remote copy andfor formalizing data in the order in which the data has been writtenfrom the host 1 (hereinafter, referred to as “write order”). Thesequence management information includes a sequence number,transfer-target information, formalization-target information, asequence number counter, transfer-target management information andformalization-target management information. The sequence number, thetransfer-target information and the formalization-target informationinclude a plurality of pieces as information to be stored in sequencemanagement entries.

The sequence number is a number provided for each unit of data suppliedfrom the host 1, and in accordance with the write order.

The transfer-target information is information for managing data to betransferred from the storage system 2 to another storage system 2,through a queue structure by use of a sequence management informationentry. Specifically, the transfer-target information is an addressindicating the position of another sequence management information entryor an address indicating the position of transfer-target managementinformation as will be described later. These addresses are stored in acorresponding area of the sequence management information entry. Usingthis information, the storage system 2 can retrieve data or the like tobe transferred next to data corresponding to one sequence managementinformation entry.

The formalization-target information is information for managing data tobe formalized, through a queue structure by use of a sequence managementinformation entry. Specifically, the formalization-target information isan address of another sequence management information entry or anaddress of formalization-target management information as will bedescribed later. These addresses are stored in a corresponding area ofthe sequence management information entry. Using this information, thestorage system 2 can retrieve data or the like to be formalized next todata corresponding to one sequence management information entry.

The sequence number counter is information to be used for recording thewrite order from the host 1.

The transfer-target management information is information by which theorder in which data will be transferred to a copy destination is managedin a copy source of asynchronous remote copy. The transfer-targetmanagement information is used as a queue header of a queue structureformed out of sequence management information entries, as follows.

First, the CPU 30 secures a sequence management information entry fordata written newly. Next, when the write source of the data is the host1, the sequence number of the written data is acquired from a sequencenumber counter. On the other hand, when the write source of the data isanother storage system 2, the sequence number included in the receiveddata is acquired. Then, the CPU 30 writes the acquired sequence numberinto the secured sequence management information entry. Incidentally,when the write source of the data is the host 1, the CPU 30 updates thevalue of the sequence number counter after acquiring the sequencenumber.

The CPU 30 refers to the queue of sequence management informationentries using the transfer-target management information as a queueheader, sequentially from the head of the queue, and connects the newlycreated sequence management information entry to the queue structure sothat the sequence numbers are arranged in the ascending order. Thisreason is that data should be transferred to a copy destinationsequentially starting with data corresponding to the sequence managementinformation entry at the head of the queue because data corresponding toa smaller sequence number was written from the host 1 earlier. Here, theupper field of the transfer-target management information in FIG. 3Bdesignates the address of the sequence management information entry atthe head of the queue, and the lower field thereof designates theaddress of the sequence management information entry at the tail of thequeue. Accordingly, when a newest sequence number is given to datawritten newly, the address in which the sequence management informationentry corresponding to the data will be stored is registered in thelower field of the transfer-target management information.

The data corresponding to a sequence manage-ment information entryconnected to a queue using the transfer-target management information asits queue header will be hereinafter referred to as “transfer-targetdata”. In addition, performing the afore-mentioned operation will bereferred to as “registering data as transfer-target data”.

The formalization-target management information is information by whichthe write order of data which has been received from a copy source buthas not yet been formalized is managed in a copy destination ofasynchronous remote copy. The formalization-target managementinformation is used as a queue header of a queue structure formed out ofsequence management information entries.

Operation similar to that for “registering data as transfer-target data”is also performed on data which has not been formalized. This is becausedata has to be formalized in accordance with its sequence numbersequentially starting with data corresponding to a sequence managementinformation entry at the head of the queue in order to secure the writeorder from the host 1. The data corresponding to a sequence managementinformation entry connected to a queue using the formalization-targetmanagement information as its queue header will be hereinafter referredto as “formalization-target data”. In addition, performing thisoperation will be referred to as “registering data asformalization-target data”. Here, the field of the transfer-targetinformation does not always match with the field of theformalization-target information because there is data which has beenformalized but has not yet been transferred.

Incidentally, the operation for registering data as transfer-target datais not performed on data in a terminal volume. It is because the datadoes not have to be transferred. In addition, the operation forregistering data as formalization-target data is not performed on datain an original volume. It is because the write order of the data issecured due to the data which has been just written from the host 1.

FIGS. 4A and 4B show an example of the details of the volume managementinformation.

The volume management information includes a remote copy target flag, asequence management information number, a newest sequence number,differential management information, registered copy destinationinformation, a copy resumption instruction flag, copy resumptionpriority, new copy source priority, registered copy source information,valid copy source information, root copy source information, etc.Further, the registered copy destination information includesinformation about a plurality of copy destinations corresponding tostorage systems 2 which will be copy destinations. The same thing isapplied to the registered copy source information.

In addition, the differential management information includes a bit map,a bit map validity flag indicating whether the bit map is in use or not,a recording start sequence number indicating the sequence number withwhich the recording of the bit map has been started, and a cumulativeupdated data amount counter indicating the cumulative updated dataamount having been counted since the recording of the bit map wasstarted. Incidentally, a plurality of bit maps and a plurality of piecesof management information (such as bit map validity flags, etc.) areprepared in advance for switching a bit map as will be described later.In addition, the differential management information also includes adifferential copy bit map to be used for differential information at thetime of differential copy. Incidentally, the details of the differentialmanagement information will be described later.

Here, the volume management information may be provided for each volumeor for each desired storage area. The storage control unit 21 can holdvolume management information for each desired storage area unit thatcan be shared among storage systems 2, and can execute remote copy foreach storage area unit.

The remote copy target flag is information indicating that a targetvolume is either a copy source or a copy destination. Specifically, itis information indicating ON when the target volume is either a copysource or a copy destination.

The sequence management information number is a number specifying thesequence management information corresponding to a volume managed by thevolume management information.

The newest sequence number indicates the most advanced sequence numberof received data if the volume is a volume of a copy destination ofsynchronous remote copy and of formalized data if the volume is a volumeof a copy destination of asynchronous remote copy.

The differential management information is information for managinginformation in which updated places in the volume (hereinafter referredto as “differential information) are recorded. The differentialmanagement information is used when the volume contents have to bematched with those in other sites due to a site's suffering fromdisaster or the like.

Here, two methods can be considered as the method for managing thedifferential information for remote copy among N sites. The firstmanagement method is a method making use of differential management inremote copy between two sites. That is, it is a method in whichdifferential management is performed in advance between sites which makeno data exchange normally but suppose that data should be matched when asite suffers from disaster. In addition, the second management method isa method which aims at differential management among desired sites. Thatis, it is a method in which update information from the host 1 startingrecording differential information is shared among a volume having thedata updated by the host 1 and all copy destinations of the data, andeach volume records the updated contents managed by the volume asdifferential information. Incidentally, data of the portions where theoriginal data differs from the updated data is called differential data.

Specific differential information will be described below by way ofexample. First, in a storage area managed by volume managementinformation, places updated after one sequence number are recorded as abit map. Further, before the start of recording as the bit map,communication is made between storage systems 2 having a copy sourcevolume and a copy destination volume for differential management so thatthe sequence numbers are matched between those volumes in prepa-rationfor the start of recording as the bit map. When the contents are matchedbetween the two volumes on the basis of this differential information,first, a volume whose newest sequence number is more advanced when thecontents are matched is specified. After that, data stored in updatedplaces indicated by the bit map for differential managementcorresponding to the volume whose newest sequence number is moreadvanced is copied to the other volume.

Alternatively, the following method may be used to prevent thedifferential copy amount from exceeding a fixed value. First, thecumulative updated data amount since the start of recording as a bit mapis recorded in the storage system 2. Then, when the cumulative updateddata amount exceeds a threshold value, communication is made betweenstorage systems having a copy source volume and a copy destinationvolume for differential management, so that a new sequence number moreadvanced than the sequence number at the time of the start of recordingof the bit map which is in use currently is shared between the storagesystems. Further, in each of the storage systems 2, recording of a bitmap different from the bit map in use is started as soon as the newestsequence number in the copy source volume and the copy destinationvolume reaches the shared sequence number, while recording of the bitmap which has been in use till then is suspended. According to thismethod, the bit map which is in use currently is switched to another newbit map so that the amount of differential data for differential copycan be reduced.

The aforementioned differential information using a bit map can beapplied to differential management among two or more desired volumes.That is, it can be used in both the aforementioned first and secondmanagement methods of differential information.

Particularly, the aforementioned differential information is used in thesecond management method of differential information as follows. Thatis, a storage system 2 having an original volume determines a sequencenumber where recording of a bit map will be started, and notifiesstorage systems 2, each having an intermediate volume or a terminalvolume, of the sequence number while transferring remote copy datathereto. As a result, the specific sequence number where recording of abit map will be started can be shared among the storage systems 2 eachhaving an original volume, an intermediate volume or a terminal volume.In addition, when the cumulative updated data amount since the start ofrecording of the bit map exceeds a threshold value, the storage system 2having an original volume determines a new sequence number more advancedthan the sequence number where recording of updated places into the bitmap in use currently was started. After that, the storage system 2having an original volume notifies the storage systems 2, each having anintermediate volume or a terminal volume, of the determined new sequencenumber while transferring remote copy data thereto. After that, each ofthe storage systems 2 each having an original volume, an intermediatevolume or a terminal volume starts recording a bit map different fromthe bit map in use currently as soon as the newest sequence number ofthe volume reaches the notified new sequence number. After that, the bitmap which has been in use till then is suspended. In addition, in thecase where it is intended to match the contents among desired ones oforiginal volumes, intermediate volumes and terminal volumes bydifferential copy in this embodiment, it will go well if the volumewhose newest sequence number is the most advanced is specified when thecontents are matched, and data recorded in updated places indicated bythe bit map of the specified volume is copied from the specified volumeto the other volumes.

In the following description of differential management, assume that theaforementioned bit map is used as differential information.

Incidentally, remote copy itself can be also executed between differentstorage systems 2 in one and the same site. For example, the remote copycan be used for changing the place where data is stored, while acceptinga read request or a write request from the host 1.

The registered copy destination information is information about a copydestination registered as a copy destination of a volume correspondingto the volume management information. Specifically, the registered copydestination information includes a registered copy destination validityflag, address information, a storage system serial number, a portnumber, a volume number, etc. (FIG. 4B). Each row in FIG. 4B is referredto as “registered copy destination information entry”. The registeredcopy destination information includes a number of registered copydestination information entries corresponding to the number of copydestinations registered. The registered copy destination validity flagis information indicating whether the registered copy destination isavailable as a copy destination or not. The address information isinformation corresponding to the protocol of the network, showing theaddress of the copy destination. The storage system serial number is anumber for specifying the storage system 2 on the network. The portnumber is a number for specifying the external connection path of thechannel interface 3 in the storage system 2. The volume number is anumber of a volume of the registered copy destination to which data of avolume corresponding to the volume management information will betransferred.

The copy resumption instruction flag is information indicating thatthere is a request to resume remote copy for the volume corresponding tothe volume management information after the remote copy is suspendedtemporarily due to a copy source's suffering from disaster or the like.When there is a request, information indicating ON is set in the entry.

The copy resumption priority is information indicating the remote copyresumption priority of a volume corresponding to the volume managementinformation after remote copy is suspended temporarily due to a copysource's suffering from disaster or the like. Remote copy is resumed infavor of a volume having a larger value of priority.

The new copy source priority is information to be used for determiningwhich copy source should be set as a new copy source when there are aplurality of valid registered copy sources.

The registered copy source information is information about a copysource registered as a copy source of a volume corresponding to thevolume management information. The configuration of the registered copysource information is similar to that of the registered copy destinationinformation. For example, the registered copy destination validity flagof the registered copy destination information corresponds to theregistered copy source validity flag of the registered copy sourceinformation.

The valid copy source information is information indicating which copysource of copy sources registered in the registered copy sourceinformation, is a current copy source.

The total copy source information is information specifying an originalvolume V1 of a volume corresponding to the volume managementinformation. The configuration of the total copy source information issimilar to that of the registered copy source information. However, thetotal copy source information includes only one entry. A storage system2 having an original volume V1 will be hereinafter referred to as atotal copy source.

The registered copy destination information, the copy resumptioninstruction flag, the copy resumption priority, the registered copysource information and the total copy source information are registeredby a user himself or herself or by a maintainance personnel or the like.

Incidentally, a plurality of pieces of sequence management informationare established in advance, and a piece of volume management informationor a set of pieces of volume management information are associated witheach piece of sequence management information. The sequence managementinformation associated with the volume management information isspecified by the sequence management information number included in thevolume management information. The aforementioned setting association isperformed by a user himself or herself or by a maintainance personnel orthe like.

Description will be made on processing when the write program 31 isexecuted in each storage system 2, with reference to FIG. 5.

The CPU 30 receives a write request (Step 501), and confirms whetherthere is a case where the write request source is the host 1 and avolume to write data thereinto (hereinafter referred to as write-targetvolume) is not a volume of a copy source. This is confirmed through theremote copy target flag (Step 502).

When the received write request is a write request to a volume having norelation to remote copy, the CPU 30 stores data into the normal area ofthe cache memory 5 (Step 503). Then, the CPU 30 makes a report of writetermination to the write request source (Step 504). Incidentally, thedata written into the normal area of the cache memory 5 is written intothe storage unit 22 by the CPU 40 asynchronously with the write requestfrom the host 1.

When there is a case other than the case where the write request sourceis the host 1 and the write-target volume is not a copy source, the CPU30 confirms whether the write request source is the host 1 or not (Step505).

When the write request source is the host 1, the CPU 30 refers to thesequence management information number of the volume managementinformation corresponding to the volume written from the host 1, andnewly secures a sequence management information entry in thecorresponding sequence management information. Then, the sequence numberof the received data is acquired from the sequence number counter, andthe sequence number is written (given) into the sequence managementinformation entry secured newly. After that, the value of the sequencenumber counter is updated.

To give the sequence number is carried out regardless of whether theremote copy is synchronous or asynchronous. The reason is as follows.That is, even if the direct copy destinations of the volume are only thecopy destinations of synchronous remote copy, there is a case wherethese copy destinations and the following destinations may executeasynchronous remote copy (Step 506).

When it is concluded in Step 505 that the write request source is notthe host 1, the CPU 30 confirms whether data related to the writerequest is differential data or not (Step 522).

In the case of differential data, the CPU 30 executes the processing onand after Step 523. The processing on and after Step 523 will bedescribed later. When the data is not differential data, the CPU 30secures a sequence management information entry corresponding to thereceived data, and stores the sequence management information includingthe sequence number received together with the data into the managementinformation memory 6 (Step 507).

After that, the CPU 30 judges whether the write-target volume is a copydestination of asynchronous remote copy or not (Step 508).

Here, description will be made on the method for storing data into thecache memory 5. When the write-target volume is a volume of a copydestination of asynchronous remote copy, the CPU 30 once saves thereceived data into the temporary area of the cache memory 5, andformalizes the data in accordance with its sequence number. In thisembodiment, the formalized data is moved from the temporary area of thecache memory 5 to the normal area thereof. When the write-target volumeis a volume of a copy source of asynchronous remote copy, in the casewhere the CPU 30 receives data for updating data which has not yet beentransferred to the copy destination of asynchronous remote copy, the CPU30 saves the data, which has not been updated and which has not beentransferred, into the temporary area of the cache memory 5, and thenstores the received data into the normal area of the cache memory 5. Onthe other hand, when the write-target volume is a volume which is a copydestination and a copy source of asynchronous remote copy, the sameprocessing as in the case where the volume is a volume of a copydestination is executed. Incidentally, processing in the case ofdifferential data is shown as different steps in FIG. 5, for the sake ofsimplification of description.

When the write-target volume is a copy destination of asynchronousremote copy, the CPU 30 has to write data in accordance with thesequence number indicating the update order from the host 1, asdescribed above. Therefore, the CPU 30 once stores the data into thetemporary area of the cache memory 5 (Step 509). Then, the CPU 30registers the stored data as formalization-target data (Step 510).Further, the CPU 30 judges whether the write-target volume is a copysource of asynchronous remote copy or not (Step 511). When it is a copysource, the CPU 30 registers the data stored in the temporary area astransfer-target data (Step 512).

When the write-target volume is not a copy destination of asynchronousremote copy, the CPU 30 judges whether the write-target volume is a copysource of asynchronous remote copy or not (Step 513). When it is a copysource, the CPU 30 stores the data into the normal area of the cachememory 5. However, when the CPU 30 receives data for updating data whichhas not yet been transferred to a copy destination of asynchronousremote copy, the CPU 30 moves the data, which has not been transferred,to the temporary area of the cache memory 5, and then stores theupdating data into the normal area (Step 514). Further, the CPU 30updates the differential management information. Specifically, a bit ata place corresponding to the data to be updated is set in a valid bitmap of the differential management information. Further, the updateddata amount is added to the cumulative updated data amount counter (Step515).

When it is concluded in Step 513 that the write-target volume is noteither a copy destination or a copy source of asynchronous remote copy,the CPU 30 stores the data into the normal area of the cache memory 5(Step 516). Further, the CPU 30 updates the differential managementinformation (specifically, in the same manner as in Step 515, sets a bitat a place corresponding to data to be updated in a bit map for thedifferential management information, and further adds the updated dataamount to the cumulative updated data amount counter) (Step 517), andexecutes the processing on and after Step 518.

On the other hand, it is concluded in Step 522 that the data isdifferential data, the CPU 30 stores the data into the normal area ofthe cache memory 5 (Step 523). After that, the CPU 30 updates thedifferential management information. Specifically, in order to recordthe progress of differential copy, a bit at a place corresponding todifferential data is cleared in a differential copy bit map in which thedifferential information received before the start of the differentialcopy is stored (Step 524).

After Steps 511, 512 and 517, the CPU 30 judges whether the write-targetvolume is a copy source of synchronous remote copy or not (Step 518).When it is a copy source, the CPU 30 transfers the data and the contentsof the sequence management information entry corresponding to the datato copy destinations of the synchronous remote copy. Incidentally, inorder to shorten the time from reception of the write request to areport of write termination made to the write request source, thesynchronous remote copy may be executed in parallel for a plurality ofcopy destinations, for example, by means of multicasting. (Step 519).

After data and so on are transferred, or when the write-target volume isnot a copy source, the CPU 30 judges whether the received data has beenregistered as either transfer-target data or formalization-target dataor not (Step 520). When the data is not registered as either, the CPU 30releases the sequence management information entry secured beforehand(Step 521) because the sequence management information entrycorresponding to the data is not required.

Finally, the CPU 30 makes a report of write termination to the writerequest source (Step 504), and terminates the processing.

On the other hand, the CPU 30 having updated the differential managementinformation in Step 524 notifies the write request source of writetermination (Step 525).

Next, the CPU 30 judges whether the write-target volume is a copy sourceor not (Step 526). When it is a copy source, the CPU 30 transfersdifferential data to copy destinations of synchronous and asynchronousremote copy (Step 527). After the differential data is transferred, orwhen the write-target volume is not a copy source, the CPU 30 terminatesthe processing. The differential copy may be carried out in parallel fora plurality of copy destinations, for example, by means of multicasting.

Description will be made on processing in the case where theasynchronous transfer program 33 is executed in a copy source ofasynchronous remote copy, with reference to FIG. 6. This program may beexecuted for each sequence management information unit. Alternatively,the program may be executed in a fixed period or in accordance with afixed target data amount. By this processing, the contents of dataupdated in a copy source of asynchronous remote copy is transferred fromthe copy source to copy destinations at any time asynchronously with awrite request to the copy source.

First, the CPU 30 selects data registered as transfer-target data (Step601).

Next, the CPU 30 transfers the selected data to the copy destinations ofasynchronous remote copy (Step 602). In order to shorten the datatransfer time to a plurality of copy destinations, the data may betransferred in parallel to a plurality of copy destination, for example,by means of multicasting.

Next, the CPU 30 clears the registration of the transferred data astransfer-target data (Step 603).

Next, the CPU 30 judges whether the transferred data has been alsoregistered as formalization-target data (Step 604). When the data hasnot been registered, the CPU 30 releases the sequence managementinformation entry corresponding to the data (Step 605) because itbecomes unnecessary to manage the transferred data. Then, the CPU 30also releases the temporary area of the cache memory 5 (Step 606) in thecase where the transferred data has used the temporary area. On theother hand, when the transferred data has been registered asformalization-target data, the CPU 30 terminates the processing as itis.

Description will be made on processing in the case where theasynchronous formalization program 41 is executed in a copy destinationof asynchronous remote copy, with reference to FIG. 7. This processingmay be executed for each sequence management information unit.Alternatively, the processing may be executed in a fixed period or inaccordance with a fixed target data amount. By this processing, data isformalized, and stored in the normal area of the cache memory 5.Incidentally, in this embodiment, there is no particular description onthe processing for transferring the data stored in the cache memory 5 tothe storage unit 22, but this processing is carried out by the CPU 40based on a known technique.

First, the CPU 40 selects data registered as formalization-target data(Step 701). In the selection method, data ranging from datacorresponding to the smallest number to data corresponding to the numberone smaller than the most advanced number of the consecutive numbers isselected from a plurality of pieces of data having consecutive sequencenumbers.

Next, the CPU 40 judges whether all the selected formalization-targetdata has been formalized or not (Step 702). When the data has not beenformalized, the CPU 40 formalizes data corresponding to the oldestsequence number out of the selected data as formalization target (Step703).

Next, the CPU 40 clears the registration of the formalized data asformalization-target data (Step 704).

Next, the CPU 40 updates the differential management information.Specifically, in the same manner as in Step 515, a bit at a placecorresponding to the formalized data is established in a valid bit mapof the differential management information. Further, the updated dataamount is added to the cumulative updated data amount counter (Step705). After that, the routine of processing returns to the processing ofStep 702.

When it is concluded in Step 702 that the data has been formalized, theCPU 40 judges whether the formalized data has been also registered astransfer-target data or not (Step 706).

When the data has not been registered, the CPU 40 clears the sequencemanagement information entry corresponding to the data (Step 707)because it becomes unnecessary to manage the data. Then, the CPU 40releases the temporary area of the cache memory 5 used by the data (Step708). On the other hand, when the formalized data is transfer-targetdata, the CPU 40 terminates the processing as it is.

FIG. 8 shows the processing procedure in each system when data istransferred to intermediate volumes V2_2 (regarded as a copy destinationof synchronous remote copy) and V2_K (regarded as a copy destination ofasynchronous remote copy), and terminal volumes V3_(K−1), V3_(N−1) andV3_N (each regarded as a copy destination of asynchronous remote copy)in the case where the data has been written from the host 1 into anoriginal volume V1_1, as shown in FIG. 1.

The storage system 2_1 having the original volume V1_1 receives a writerequest from the host 1 (Step 1201), gives a sequence number to dataaccompanying the write request (Step 1202), and stores the data giventhe sequence number into the cache memory 5 (Step 1203). Further, thedata stored in the cache memory 5 is registered as transfer-target datafor the intermediate volume V2_K which is a copy destination ofasynchronous remote copy (Step 1204). After that, the data and thesequence number stored in the cache memory are transferred to thestorage system 2_2 having the intermediate volume 2_2 which is a copydestination of synchronous remote copy (Step 1205).

The storage system 2_2 stores the received data into the cache memory 5(Step 1206), and stores the sequence number received together into themanagement information memory 6. Further, the storage system 2_2registers the received data as transfer-target data for the terminalvolumes V3_(K−1) and V3_N which are copy destinations of asynchronousremote copy (Step 1207). In response to a reply of data reception fromthe storage system 2_2, the storage system 2_1 makes a report of writetermination to the host 1 (Step 1208). Then, the write request from thehost 1 is terminated.

In the storage system 2_2, the data registered in Step 1207 is selectedas transfer target, and the data and the sequence number are transferredto the terminal volumes V3_(K−1) and V3_N at predetermined timing (Step1209).

Each of the storage systems 2_(K−1) and 2_N having the terminal volumesV3_(K−1) and V3_N stores the received data into the temporary area ofthe cache memory 5 (Step 1210, 1212), and further registers the receiveddata as formalization-target data (Step 1211, 1213). After that, in eachof these storage systems 2, the registered data is selected asformalization-target data, and formalized (Step 1214, 1215).

On the other hand, the storage system 2 _(—)1 selects the dataregistered in Step 1204, as transfer target, and transfers the data andits sequence number to the intermediate volume V2_K which is a copydestination of asynchronous remote copy (Step 1216).

The storage system 2_K stores the received data into the temporary areaof the cache memory 5 (Step 1217), registers the received data asformalization-target data (Step 1218), and registers the received dataas transfer-target data for the terminal volume V3_(N−1) which is a copydestination of asynchronous remote copy (Step 1219). After that, in thestorage system 2_K, the registered data is selected asformalization-target data, and formalized (Step 1220).

The processing from Step 1221 to Step 1224 executed by the storagesystem 2_K storing the intermediate volume V2_K and the storage system2_(N−1) storing the terminal volume V3_(N−1) is similar to theprocessing from Step 1209 to Step 1215.

FIG. 9 is a flow chart for explaining the processing when a failure of acopy source has been detected by the CPU 30 in a storage system 2 havingan intermediate volume V2 or a terminal volume V3.

First, the CPU 30 detects a failure of a copy source. For example, thestorage system 2 concludes a failure of a copy source when time out orerror retry out occurs in data transfer, confirmation of a transfer pathexecuted in a fixed frequency between a copy source and a copydestination, or the like (Step 801).

Next, the CPU 30 selects a volume which has not yet been examined as towhether there occurs a failure in a copy source therefor, on the basisof information in the management information memory 6 (Step 802).

Next, the CPU 30 examines whether there occurs a failure in a copysource corresponding to the selected volume. Specifically, it is judgedon the basis of whether the copy source registered as valid copy sourceinformation is identical to the copy source having a failure or not(Step 803). When the copy source corresponding to the selected volumehas a failure, the CPU 30 starts the remote copy suspension program 32for the selected volume. When the remote copy suspension program isstarted, remote copy suspension processing is performed. The processingwill be described later (Step 804).

Next, the CPU 30 examines whether the processing from Step 802 to Step804 has been executed on all the remote copy target volumes or not (Step805). When it has been executed, the processing of Step 806 is executed.When it has not been executed, the routine of processing returns to Step802, executing the processing therefrom.

Next, the CPU 30 judges whether remote copy for all the volumes having afailure in their copy source has been suspended (Step 806). When it hasnot been suspended, the CPU 30 waits for a predetermined time (Step807), and executes the processing of Step 806 again.

When the remote copy has been suspended, the CPU 30 checks the volumemanagement information so as to select a volume as a volume for whichthe remote copy should be resumed. As the volume, a volume whose remotecopy has not yet been resumed, whose copy resumption instruction flag isON, and whose copy resumption priority is the highest, is selected froma set of volumes having a failure in their copy source. This is becausethe remote copy for data having higher priority is resumed more earlierand more surely.

Incidentally, for volumes having low priority, their copy resumptioninstruction flags may be set to be OFF in advance. In this case, remotecopy for the volumes is suspended as soon as there occurs a failure. Inaddition, for volumes having middle priority, their copy resumptioninstruction flags may be set to be ON, and their copy resumptionpriorities may be set to be low. Thus, such a case is avoided thatremote copy for such a volume is resumed to increase the load on thestorage system 2 so that remote copy for a volume having higher prioritycannot be resumed. Further, an intermediate volume or a terminal volumein a site closer to an original volume may be set to have a higher valueof copy resumption priority so that required minimum multiplicity forremote copy can be secured (Step 808).

Next, the CPU 30 starts the remote copy resumption program for theselected volume. The remote copy resumption program will be describedlater (Step 809).

Next, the CPU 30 judges whether a remote copy resumption notificationhas been received from a new copy source. Specifically, the CPU 30judges whether a remote copy resumption notification in Step 1103 inFIG. 12 in the differential copy program executed by a storage system 2of a new copy source as will be described later has been detected ornot. Here, the reason why the CPU 30 does not wait till the differentialcopy processing with the new copy source is terminated is as follows.That is, there is a possibility that it takes long time till thedifferential copy processing of a volume performed by the start of thecopy resumption program is terminated. Therefore, it will takes muchtime to resume remote copy if the CPU 30 waits till the differentialcopy processing is terminated. In addition, once remote copy is resumed,there is no fear that the remote copy which has already been resumed isinterrupted again by the influence of remote copy resumed newly.Therefore, only after remote copy for one volume has been resumed,remote copy can be resumed with intended priority even if remote copyresumption processing for a new volume is kept up. Alternatively, as asimpler method, based on the fact that the remote copy has been resumedby the start of the remote copy resumption program in Step 809, it maybe concluded that a remote copy resumption notification has beenreceived from the copy source (Step 810). When a remote copy resumptionnotification has not been received, the CPU 30 waits for a predeterminedtime (Step 811), and executes the processing of Step 810 again.

Next, the CPU 30 examines whether the processing from Step 808 to Step811 has been executed on all the volumes having a failure in their copysource or not (Step 812). When the processing has been executed, theroutine of processing is terminated. When the processing has not beenexecuted, the processing on and after Step 808 is executed again.

Incidentally, not when a failure in a copy source has been detected butwhen an overload notification indicating the difficulty of keeping upremote copy processing due to increase in load on the copy source or thelike has been received from the copy source, the copy source may bechanged in response thereto. A specific example of the load on a copysource includes, for example, the case where the rate of utilization ofthe temporary area of the cache memory 5 exceeds a predeterminedthreshold value in asynchronous remote copy.

Alternatively, the copy source may be changed in response to aninstruction from the maintenance terminal 23 by a user himself orherself or by a maintainance personnel or the like. Here, all thestorage systems 2 can identify a current copy source, a total copysource and a current copy destination for each remote copy target volumeon the basis of the information shown in FIGS. 4A and 4B. Accordingly,only if the user himself or herself, the maintainance personnel or thelike issues an instruction to a desired storage system 2 having one ofan original volume, an intermediate volume and a terminal volume throughthe maintenance terminal, the host 1 or the like, an instruction tochange the copy source for an original volume, and all the intermediatevolumes and terminal volumes which will be remote copy destinations ofthe original volume can be issued.

Further, a management site having a management terminal may be providednewly so that information such as total copy source informationbelonging to all the storage systems 2 each having an original volume isregistered in the management terminal. In this case, using themanagement terminal, the user can issue an instruction to change copysources for all the original volumes belonging to the storage systems 2connected to the network, and all the intermediate volumes and theterminal volumes in copy destinations of all the original volumes.Incidentally, an existing site may be used as the management site, thatis, one host 1 may be used as the management terminal.

In addition, when storage systems 2 of other sites gain access to themanagement terminal, for example, after a copy source of another copysource can detect a failure in the latter copy source, and gives anotification of the failure to copy destinations through the managementterminal. The copy destinations may change their copy source in responseto this notification.

FIG. 10 is a flow chart for explaining the processing procedure of theremote copy suspension program 32 started in Step 804 in FIG. 9.

First, the CPU 30 examines whether a volume instructed to suspend remotecopy is a copy destination of synchronous remote copy or a copydestination of asynchronous remote coy (Step 901). In the case ofasynchronous remote copy, the CPU 30 formalizes any data that can beformalized (Ste 902).

In the case of synchronous remote copy, or after finishing formalizingdata in the case of asynchronous remote copy, the CPU 30 judges whetherthere are copy destinations in the volume which will be a target ofremote copy suspension or not (Step 903).

When there are copy destinations in the volume (corresponding to anintermediate volume V2), the CPU 30 gives a notification of remote copysuspension to all the direct copy destinations of the volume (Step 904).When there is no copy destination in the volume (corresponding to aterminal volume V3), the CPU 30 terminates the processing.

Next, the CPU 30 judges whether a reception report of the remote copysuspension notification has been received from each of the direct copydestinations of the volume (Step 905).

When there is no report, the CPU 30 waits for a predetermined time (Step906), and executes the processing of Step 905 again. When there is areport, the CPU 30 terminates the remote copy suspension processing.

On the other hand, a storage system 2 receiving the aforementionedremote copy suspension notification or the like issued in Step 904executes the volume notification transfer program 34, so as to executeprocessing similar to that from Step 901 to Step 906. In this case, thestorage system 2 receiving the remote copy suspension notificationsuspends remote copy in the newest sequence number the same as that inthe volumes taking part in the remote copy suspension notification.

Then, when a remote copy suspension notification reception report hasbeen received from each of the direct copy destinations of the volume,the storage system 2 transmits the remote copy suspension notificationreception report to the reception source of the remote copy suspensionnotification.

Incidentally, the CPU 30 executes the volume notification transferprogram 34 so as to transfer a remote copy resumption notification, adifferential copy notification and so on as well as the remote copysuspension notification to other storage systems 2. However, differentlyfrom the processing for the remote copy suspension notification, Step901 and Step 902 are not executed for the remote copy resumptionnotification, the differential copy notification and so on.

FIG. 11 is a flow chart for explaining the remote copy resumptionprocessing of the remote copy resumption program 36 started in Step 809in FIG. 9.

First, the CPU 30 refers to the registered copy source validity flag ineach piece of registered copy source information of volumes having afailure in their copy source, and judges whether there are validregistered copy sources or not (Step 1001). When there is no validregistered copy source, the CPU 30 terminates the processing because thecopy source cannot be changed to keep up the remote copy.

When there are valid registered copy sources, the CPU 30 refers to thenew copy source priority so as to select a copy source having highpriority. The new copy source priority is set in advance for eachregistered copy source by the user. In addition, the number ofregistered copy sources may be increased or decreased in accordance withthe priority of data to be stored in a volume so that remote copy for ahigh-priority volume having a large number of registered copy sourcescan be kept up with high probability as soon as there occurs a sitefailure. In addition, since changing a copy source results in a new loadon a new copy source, no copy source may be registered for any volumehaving low priority, and remote copy may be canceled as soon as thereoccurs a failure (Step 1026).

Next, the CPU 30 judges whether the path between the selected validregistered copy source and the storage system 2 of its own has beenformed or not. Specifically, the CPU 30 refers to volume managementinformation of other volumes sequentially, and concludes that the pathhas been formed if the storage system number and the port number of oneof the copy sources registered in valid copy source information of thevolume management information of other volumes coincide with those ofthe selected valid registered copy source (Step 1002).

When the path has not yet been formed, the CPU 30 makes communicationwith the storage system 2 indicated by the address information of theregistered copy source, and forms the path thereto (Step 1003).

After the path is formed, or when the path has already been formed, theCPU 30 judges whether the volumes having a failure in their copy sourcecan be registered in the registered copy destination of the volumemanagement information corresponding to a volume belonging to a new copysource or not (Step 1027). When the number of registered copydestinations of the volume management information corresponding to thevolume belonging to the new copy source has already exceeded a thresholdvalue, the CPU 30 cancels the registration of the volumes belonging tothe new copy destinations as copy destinations in order to prevent thenew copy source from being overloaded, makes the registered copy sourcevalidity flag invalid (Step 1028), and executes the processing of Step1001 again.

When each volume having a failure in its copy source can be registeredas a registered copy destination of the volume belonging to the new copysource, the CPU 30 makes judgement using the total copy sourceinformation as to whether the original volume V1 of the volume belongingto the new copy source coincides with the original volume of the volumeas a target of remote copy resumption (Step 1004). When the requirementof Step 1004 is not satisfied, the CPU 30 terminates the processingbecause the CPU 30 cannot change the copy source to keep up the remotecopy.

When the requirement of Step 1004 is satisfied, the CPU 30 changes thevalid copy source information corresponding to the volume as a target ofremote copy resumption into the information about the new copy source.At this time, the copy source is switched to the new copy source (Step1005).

When the copy source having a failure is a total copy source, there isno registered copy source. Therefore, the CPU 30 terminates theprocessing. In this case, when remote copy between the total copy sourceand a copy destination is synchronous remote copy, data in the totalcopy source coincides with data in the copy destination. Thus, theoperation of the total copy source can be turned over directly to thecopy destination. On the other hand, when remote copy between the totalcopy source and the copy destination is asynchronous remote copy, it isinevitable that data which has not yet been transferred from the totalcopy source to the copy destination will be lost. However, theconsistency of data in the copy destination can be secured (the data inthe copy destination can be restored into the state of the total copysource at a past time). Thus, the operation can be resumed in the copydestination by use of the data with secured consistency (Step 1006).

Next, the CPU 30 makes a request to the new copy source for a newestsequence number (Step 1007).

The CPU 30 having received the newest sequence number judges whether thereceived newest sequence number is more advanced than the newestsequence number of the volume as a target of remote copy resumption ornot (Step 1008).

When it is not more advanced, the CPU 30 waits for a predetermined time(Step 1009), and executes the processing of Step 1007 again. This isprocessing for waiting till the updated state of the volume which willbe a copy source in differential copy is more advanced than the volumesuspended in remote copy in order to execute differential copy.

When it is more advanced, the CPU 30 makes a request to the new copysource for resumption of remote copy between the volume as a target ofremote copy resumption and the volume belonging to the new copy source,and execution of differential copy. In response to this request, thedifferential copy program 35 is executed in the storage system 2 in thenew copy source. The differential copy program 35 will be describedlater (Step 1010).

Next, the CPU 30 judges whether a remote copy resumption notificationand differential information have been received from the new copy sourceor not. The remote copy resumption notification and the differentialinformation to be received by the CPU 30 correspond to a replytransmitted to each copy destination by the storage system in the newcopy source executing the differential copy program 35. (Step 1011).

When they have not yet been received, the CPU 30 waits for apredetermined time (Step 1012), and executes the processing of Step 1011again. When they have been received, the CPU 30 judges whether thevolume as a target of remote copy resumption has direct copydestinations or not (Step 1013). When the volume has direct copydestinations, the CPU 30 notifies the direct copy destinations of theremote copy resumption and the differential information. Each storagesystem 2 notified of the information executes the volume notificationtransfer program 34 so as to transfer the notification of the remotecopy resumption and the differential information to terminal volumes(Step 1014). Incidentally, the transferred differential information isstored in a differential copy bit map by the CPU 30 in each storagesystem 2.

Next, the CPU 30 judges whether a remote copy resumption completionreport and a differential information reception report have beenreceived from each of the direct copy destinations of the volume or not(Step 1015). When the reports have not yet been received, the CPU 30waits for a predetermined time (Step 1016), and executes the processingof Step 1015 again.

When the reports have been received, the CPU 30 concludes that theremote copy resumption is completed, and carries out the reception of adifferential copy termination notification from the new copy source andthe transfer of differential copy termination to the copy destinationsof the volume to resume remote copy therefor in the procedure similar toStep 1011 to Step 1016. Incidentally, as soon as the CPU 30 of the copydestination in the differential copy receives the differential copytermination notification, the CPU 30 refers to the differentialinformation received before the start of the differential copy andcontinuously updated during the differential copy, and judges whetherall the differential data has been received or not. When there isdifferential data that has not been received, the CPU 30 inserts theinformation about the differential data that has not been received, intoa differential copy termination report, so as to make a request to thecopy source for retransmission of the differential data. Specifically,the CPU 30 judges whether there is a bit that has not yet been clearedor not, in the differential copy bit map. When there is a bit that hasnot yet been cleared, the CPU 30 inserts the position information of thedifferential data corresponding to the bit in question into thedifferential copy termination report (Step 1017 to Step 1022).

Next, the CPU 30 judges whether a differential data retransmissionrequest is included in the differential copy termination report or not(Step 1023). When there is a retransmission request, the CPU 30 makes arequest to the new copy source for retransmission of the differentialdata (Step 1024), and executes the processing on and after Step 1017again. When there is no retransmission request, the CPU 30 reports thecompletion of the differential copy to the new copy source (Step 1025),and terminates the processing.

FIG. 12 is a flow chart for explaining the processing when thedifferential copy program 35 is executed in the storage system 2 of thenew copy source.

First, in response to the request in Step 1010 in FIG. 11, the CPU 30registers the information about the volume having received the remotecopy resumption notification into a registered copy destinationinformation entry of the volume which will be a new copy source, andsets the registered copy destination validity flag (Step 1011).

Next, the CPU 30 determines differential information between the volumewhich will be a new copy source and the volume having received thedifferential copy notification (volume of the copy destination).Specifically, the CPU 30 determines the differential information bycopying a bit map of differential management information at the time ofreceiving a notification of the volume which will be a new copy source,to a differential copy bit map area (Step 1102). Incidentally, themethod for determining the differential information differs inaccordance with how to hold the differential management information.

Next, the CPU 30 gives a notification of the remote copy resumption andthe determined differential information to the storage system 2 havingtransmitted the remote copy resumption notification. This notificationis detected in Step 1011 (Step 1103).

Next, the CPU 30 transfers the differential data to the copy destination(Step 1104). After that, the CPU 30 judges whether transferring all thedifferential data is finished or not (Step 1105).

When transferring is not finished, the CPU 30 transfers the differentialdata to the volume having received the differential copy notification(copy destination) (Step 1106), and executes the processing of Step 1105again. When transferring is finished, the CPU 30 gives a notification ofthe termination of the differential copy to the reception source of thedifferential copy notification. This notification is detected in Step917 (Step 1107).

Next, the CPU 30 judges whether the report from the reception source ofthe differential copy notification is retransmission of the differentialdata or not (Step 1108).

When the report is a retransmission request, the CPU 30 retransmits therequested differential data (Step 1109), and executes the processing ofStep 1107 again. When the report is not a retransmission request, theCPU 30 confirms that the report indicates the differential copycompletion, and terminates the processing.

FIG. 13 is a diagram showing a specific example of the processingprocedure in which a failure in a copy source is detected in a storagesystem 2 having an intermediate volume V2, with the result that a newcopy source is selected, and remote copy is resumed. As illustrated, thestorage system 2 having detected a failure of the copy source executesthe processing for detecting the copy source failure, the remote copyresumption program and the remote copy suspension program in parallel.

The storage system 2 having detected the copy source failure (Step 1301)starts the execution of the remote copy suspension program 32 for eachof volumes having a failure in their copy source (Step 1302).

For one of the volumes having a failure in their copy source, thestorage system 2 formalizes any formalizable data of asynchronous remotecopy (Step 1303), and gives a notification of remote copy suspension todirect copy destinations of the volume (Step 1304).

On the other hand, after remote copy for all the volumes having afailure in their copy source is suspended (Step 1305), the storagesystem 2 selects a volume having higher priority as a volume to resumeremote copy therefor (Step 1306), and starts the execution of the remotecopy resumption program 36 for the volume in question (Step 1307).

The storage system 2 executing the remote copy resumption program 36selects a registered copy source having higher priority as a new copysource (Step 1308), and forms a path thereto if necessary (Step 1309).Further, the storage system 2 confirms the coincidence between thevolume belonging to the new copy source with the original volume of thevolume in question to resume remote copy therefor (Step 1310) andchanges the valid copy source into the new copy source so as to switchthe current copy source to the new copy source (Step 1311). Next, thestorage system 2 waits till the newest sequence number of the volumebelonging to the new copy source is more advanced than the newestsequence number of the volume in question to resume remote copy therefor(Step 1312), and makes a request to the new copy source for theresumption of remote copy and the execution of differential copy (Step1313).

As soon as the storage system 2 as a new copy source receives therequest for the resumption of remote copy and the execution ofdifferential copy in Step 1313, the storage system 2 starts thedifferential copy program 35.

The storage system 2 as a new copy source registers the volume inquestion to resume remote copy therefor as a copy destination of thevolume belonging to the new copy source (Step 1314), and determinesdifferential information (Step 1315). Further, the storage system 2 as anew copy source gives a notification of the resumption of remote copyand the determined differential information to the storage system 2 as arequest source of the resumption of remote copy and the execution ofdifferential copy in Step 1313 (Step 1316).

After that, the storage system having detected the copy source failureexecutes the remote copy resumption program 36, and gives thenotification of the resumption of remote copy and the determineddifferential information notified in Step 1316, to direct copydestinations of the volume in question to resume remote copy therefor(Step 1317).

On the other hand, the storage system 2 detects the resumption of remotecopy notified in Step 1316, selects a volume having higher priority as avolume to resume remote copy therefor (Step 1318), and starts theexecution of the remote copy resumption program 36 for the selectedvolume (Step 1319).

The storage system as a new copy source transfers differential data tothe storage system having detected the copy source failure (Step 1320).The storage system 2 having detected the copy source failure andreceived the differential data executes the write program 31 so as tofurther transfer the differential data to the direct copy destinationsof the volume in question to resume remote copy therefor (Step 1321).

In addition, in parallel with the transfer of the differential data, thestorage system as a new copy source executes the write program 31 so asto transfer the received remote copy data to the copy destinationsincluding the storage system 2 having detected the copy source failure(Step 1322). The storage system 2 having detected the copy sourcefailure executes the write program 31 so as to further transfer theremote copy data to the direct copy destinations of the volume inquestion to resume remote copy therefor, synchronously or asynchronouslywith the reception of the remote copy data (Step 1323).

Further, as soon as the transfer of the differential data is terminated,the storage system as a new copy source gives a notification of thetermination of the differential copy to the request source of theresumption of remote copy and the execution of the differential copy inStep 1313 (Step 1324).

The storage system having detected the copy source failure executes theremote copy resumption program 36 so as to give the notification of thetermination of the differential copy notified in Step 1324, to thedirect copy destinations of the volume in question to resume remote copytherefor (Step 1325).

On giving the notification of the termination of the differential copy,the storage system 2 having detected the copy source failure terminatesthe remote copy resumption program 36 and the differential copy program35 for the volume in question to resume remote copy therefor.

Next, description will be made on the site restoring procedure formaking a site restored from its suffered disaster state perform remotecopy operation again.

First, each unit included in the site, for example, the storage system 2is restored to its state before disaster by man power or the like.Specifically, a new facility or a restored facility is secured, and thehost 1 and the storage system 2 are installed and connected by man poweror the like. In addition, a communication path with other sites issecured.

Next, in each storage system 2 in the restored site, an intermediatevolume or a terminal volume out of remote copy target volumes before thesuspension of remote copy caused by disaster or the like, isreconstructed by remote copy from a storage system 2 which was a copysource before remote copy was suspended. The procedure has the followingoutline.

First, in each storage system 2 in the restored site, a transfer pathwith the storage system 2 which was a copy source previously isestablished. Next, a volume belonging to the restored storage system 2is set as a copy destination volume (intermediate volume or terminalvolume) again in a volume of the storage system 2 which was a copysource. Then, transferring of the data as a whole included in the volumeset as a copy source again, and the updated contents thereof is started.The setting of the copy destination volume may be the same as or quitedifferent from before the suspension of remote copy.

Next, when the volume having contents restored by remote copy has avolume (that is, an intermediate volume) set to have a copy destinationvolume, the storage system 2 gives a notification of the restoration ofthe storage system 2 to the storage system 2 set as a copy destination.

The copy destination having received the notification of the restorationof the storage system 2 executes the remote copy suspension program 32and the remote copy resumption program 36, and switches the copy sourceto the restored storage system 2.

From above, it is possible to make the site restored from disasterperform the operation of remote copy again.

According to the present invention, remote copy can be resumed chieflyamong storage systems disposed in three or more N sites immediatelyafter any site suffers from disaster.

The aforementioned description was made on an embodiment, but thepresent invention is not limited thereto. It is apparent to thoseskilled in the art that various changes and modifications can be made onthe present invention without departing the claimed scope and the spiritof the present invention.

1. A storage system connected among a plurality of storage systems,comprising: a storage control unit; and a storage unit connected to saidstorage control unit; wherein said storage control unit includes amanagement information memory for storing management information formanaging data, said management information including a target flag;wherein said management information memory has first and second sourceinformation about said plurality of storage systems; wherein saidstorage control unit receives data sent from a first storage system inresponse to said target flag being set when said first sourceinformation includes information of said first storage system, andperforms processing for receiving, by changing said second sourceinformation from said first to second storage system, data from saidsecond storage system in response to said target flag being set whensaid first source information includes information of said secondstorage system when data cannot be received from said first storagesystem; and wherein when said storage control unit receives informationof restoration of said first storage system, said storage control unitchanges said second source information from said second to said firststorage system to receive data from said first storage system.